1. Field of the Invention
The invention relates to semiconductor memories and in particular to timing circuits for controlling reading and writing in such memories.
2. Prior Art
In semiconductor memories, such as static and dynamic random-access memories (RAMs), a plurality of control or timing signals are typically used to access memory cells. Often, to access a cell for reading or writing, several control signals are needed, each occurring one after the other to, for example, precharge nodes, provide isolation, activate sense amplifiers, etc. The timing of these signals is critical and becomes more critical where the memory is operated at faster rates.
In early semiconductor RAMs, control signals were generated off-chip. In current memories, the control signals are generated on-chip; see, for example, U.S. Pat. No. 3,778,784. In the metal-oxide-semiconductor (MOS) memory described in the '784 patent, a timing cell is used in the timing circuits. This cell compensated for process variations which would otherwise substantially affect the timing of these timing signals.
In U. S. Pat. No. 4,087,704, another on-chip control signal arrangement is described for a dynamic RAM where the output of one control signal generator is used to initiate or trigger the next control signal generator. This arrangement assures that a delay occurs between each of the signals and that the signals occur in a predetermined order.
It is also common in semiconductor memories to employ dummy cells. These cells, for example, provide loading or charge in an amount equal to that of a real cell in the array. The dummy cells are fabricated on the same die or substrate as the true memory cells, hence, their characteristics are effectively the same as the real cells (independent of process variations). An example of dummy cells is described in U.S. Pat. No. 3,959,781.
As will be seen, the present invention employs models of actual circuit elements to provide delays in a timing circuit. The output of one model is used in some cases to activate another model, for example, a word line model initiates reading in a cell read model.